Communications systems are well known and consist of many types including land mobile radio, cellular radiotelephone, personal communications systems, and other communications system types. Within a communications system, transmissions are conducted between a transmitting device and a receiving device over a communication resource, commonly referred to as a communication channel.
One type of communications system known in the art is a spread-spectrum system. In spread-spectrum systems, all communication signals are transmitted simultaneously in the same transmission bandwidth. This modulation technique spreads the frequency spectrum or information bandwidth of each communication signal using a spreading code. Several different spreading codes are used in spread-spectrum systems including, but are not limited to, pseudo noise (PN) codes and Walsh codes. The codes used for spreading have low cross-correlation values and are unique to each simultaneous communication signal. Spreading codes can be used to separate communication signals from one another in a spread-spectrum communications system. Therefore, spreading codes can effectively limit the number of communication signals that can be simultaneously transmitted via the overall transmission bandwidth. One of the main parameters in determining the spreading code for a given communication signal is the processing gain, which is the ratio of the transmission bandwidth to the information bandwidth. When the processing gain is high, the spreading code does not have to separate communication signals as far apart and more communication signals can be simultaneously transmitted. This correlation between processing gain and spreading codes demonstrates how processing gain can also impact the number of communication signals simultaneously transmitted in a system. Since it is advantageous to keep the processing gain as high as possible, it is equally advantageous to minimize the information bandwidth of individual communication signals, the information bandwidth being inversely proportional to the processing gain.
Spread-spectrum systems include wireless cellular networks that communicate messages to and from mobile devices through a wireless cellular infrastructure. Several types of wireless cellular networks are known in the art and are generally grouped in terms of first generation (1G), second generation (2G) and third generation (3G) technology.
1G wireless cellular networks are based on analog technology. The most widely deployed 1G wireless cellular networks are the advanced mobile phone system (AMPS), Nordic mobile telephone (NMT), and total access communications system (TACS). 2G wireless cellular networks are based on digital technology. 2G wireless cellular networks include IS-95 or cdmaOne, IS-136 or digital AMPS (DAMPS), global system for mobile (GSM), and personal digital cellular (PDC) systems. IS-95 utilizes code division multiple access (CDMA) as its air interface communication protocol. Alternatively, IS-136, GSM, and PDC utilize time-division multiple access (TDMA). 1G and 2G wireless cellular networks currently provide voice services and low-rate data services.
3G is the next generation of wireless cellular technology with its primary focus on seamlessly evolving 2G systems to provide high-speed data services to support various data and multimedia applications, such as web page browsing. To preserve the existing wireless infrastructure, it is preferable for 3G systems to be compatible with existing voice and low-rate data capabilities of 1G and 2G systems. International mobile telecommunications in the year 2000 (IMT-2000) is the 3G specification under development by the International Telecommunications Union (ITU) that will provide standardized requirements for enhanced voice and data services over next generation wireless networks. Proposed 3G wireless cellular networks include cdma2000 and wideband CDMA (WCDMA). Universal mobile telecommunications system (UMTS) is often used synonymously with IMT-2000 and is also frequently used when referring specifically to WCDMA.
The generalized architectural framework of a wireless cellular network is based on the geographic placement of a plurality of base transceiver stations (BTSs), each BTS creating a geographic coverage area known as a cell. A BTS communicates with wireless mobile devices within its cell and such communications are maintained by the wireless cellular network as the wireless mobile devices move geographically from cell to cell. In addition to multiple BTSs, the wireless infrastructure of a wireless cellular network includes at least one base station controller (BSC), at least one selector distributor unit (SDU), and at least one mobile switching center (MSC). Within a wireless cellular network, a forward message refers to a message transmitted by cellular infrastructure equipment for reception by a wireless mobile device. A reverse message refers to a message generated by a wireless mobile device, such as a mobile cellular telephone. The typical wireless cellular communications system communicates with other communications systems, such as a public switched telephone network (PSTN), via the MSC. External interfaces, such as a PSTN interface, provide the wireless cellular network with access to individual computers and distributed computer networks, including local area networks (LANs), wide area networks (WANs), intranets, internets, the Internet, and any other distributed processing and/or storage networks. Within the wireless infrastructure, the MSC communicates with one or more BSCs. A BSC communicates with multiple BTSs. BTSs communicate, over an air interface via a radio frequency (RF) channel, with wireless mobile devices operating within their respective coverage areas through forward and reverse links. An SDU is coupled to the MSC, one or more BSCs, and multiple BTSs. The SDU performs forward link air interface frame distribution to the BTSs.
In a spread-spectrum system that provides wireless access to individual computers and distributed computer networks, it is possible that a given data transmission will be limited in forward link throughput by the throughput of the computer equipment infrastructure, rather than by the wireless link speed. In other words, the bottleneck link speed and information bandwidth of the data transmission is limited by the computer equipment infrastructure. Under such circumstances, transmission bandwidth resources in the spread-spectrum communications system are wasted if a link speed higher than the bottleneck link speed is assigned to the forward link by the wireless infrastructure. In other words, the wireless link speed should not be much faster than the bottleneck link speed established by certain components of the computer equipment infrastructure.
Consequently, a need exists for an apparatus and method which optimizes the data rate for forward link data transmissions in a spread-spectrum communications system in a manner which maximizes usage of the transmission bandwidth while also maintaining a sufficient quality of service (QoS) to the wireless mobile device.